Older designs of dimmer switches used variable resistors to control the voltage delivered across a load. A disadvantage of that approach is that energy not delivered to the load is instead converted into heat in the variable resistor, which results in the switches being inefficient and potentially dangerous.
More modern dimmers work by reducing the rms voltage across the load by switching current to the lighting load on and off many times per second. A mains AC circuit provides a voltage that varies sinusoidally from a peak positive voltage through zero voltage to a peak negative voltage. Modern dimmers work by chopping up each period of the sine wave of the AC signal into a portion in which the load is on and a portion in which it is off. One approach is to turn current to the load off whenever the AC current passes through zero current (i.e. whenever the current in the circuit reverses direction), and then to turn it back on at a voltage determined by the setting of a control knob or the like. Another approach is to turn current to the load on whenever the AC current passes through zero current, and then to turn it off at a voltage determined by the setting of a control knob or the like. In either case, the amount of energy supplied to the load increases or decreases as the load is turned on for more or less of the AC cycle; in other words, for a lighting load, the brightness of the light varies with changes in the proportion of the sine wave of the AC supply for which the light is switched on (the duty cycle).
The desired switching behaviour can be achieved for example by providing a knob-controlled variable resistor and a firing capacitor, which in combination control the semiconductor switch, with the semiconductor switch being switched when the voltage across the capacitor exceeds a threshold value, and the time taken to reach the threshold voltage being controlled by a variable resistor. Thus, for example, in some prior-art dimmers, the semiconductor switch is a triac, and the gate of the triac is connected between the firing capacitor and the variable resistor.
In order to achieve dimming by controlling the duty cycle, it is usually necessary to use a switch that is capable of switching at a frequency similar to the AC mains frequency (e.g. 50 Hz or 60 Hz). It is usual to use a semiconductor switch for that purpose. The most common choice of semiconductor switch is a triac, but it is also known to use a field effect transistor (FET), for example a metal-oxide-semiconductor FET (MOSFET). Dimmers in many lighting control systems use MOSFET switches as they have the lowest voltage drop compared with other devices such as IGBTs, thyristors and BJTs which drop around 2 V; the drop across a MOSFET is the load current multiplied by the drain-source resistance when the MOSFET is on. MOSFETs with a drain-source resistance of 50 milliohms are readily available at low prices. The power loss for a 1 KW load in such a MOSFET is less than 2.5 Watt.
Nevertheless, there is a finite voltage drop across a MOSFET switch, and as the current delivered to the light or other load flows through the switch for at least part of each cycle, the resultant power is dissipated as heat, resulting in an increase in the temperature of the switch. When the load increases beyond the rated value, the temperature of the switch can go beyond safe limits. Incorrect wiring during installation can cause a short circuit of the MOSFET load, and some loads (e.g. halogen lamps) can fail into a short-circuit mode. Consequently, in some circumstances, the MOSFETs can see a shorted load with AC mains applied across it, and the current generated during such a fault can be very large, and can easily damage the MOSFETs.
To protect the MOSFETs against excessive current due to short-circuiting of the load, it is necessary to shut down the MOSFET before the current and heat dissipation become unsafe. Prior-art designs for achieving shut-down, which typically use integrated circuits and current-sensing elements, are relatively complex and expensive.
WO03/005550A1 describes an apparatus in an electronic control system that allows two or three wire operations. Over-current circuitry senses when the current through MOSFETs has exceeded a predetermined current threshold, and then turns off the MOSFETs, so they do not exceed the safe operating area curve of the MOSFET. Latching circuitry is employed to keep the protection circuitry in effect even after a fault condition has been cleared. The protection circuitry output is said to be desirably configured such that it can bypass and override the normal turn off impedance and act as a lower resistance across the gates and sources of the MOSFETs. The current protection is provided by sensing a voltage across a sense resistor, and comparing the voltage generated with the base-to-emitter voltage of two transistors to initiate a current limit action. The WO03/005550A1 circuit uses an integrated circuit (IC)-based comparator, which requires a bias supply (with an attendant power requirement) and reference.
The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved dimmer for controlling equipment, for example lighting equipment.